The invention relates to aircraft whose wings can exhibit buffeting under certain flight conditions.
The lfit surfaces of aircraft may become subject to phenomena associated with the coupling of the structure with the surrounding aerodynamic flow, and particularly flutter and buffeting.
Flutter is an oscillatory motion resulting from structural instability related to vibrational behaviour or airplane structures, particularly wings, in certain frequency ranges. It results from coupling of torsional and bending oscillation at different frequencies which induces regenerative oscillation of increasing amplitude at a predetermined frequency.
The response of an unstable structure may be computed since apparition of flutter is determined by structural and aerodynamical parameters which may be taken into account. It is also possible to determine a critical speed above which the aerodynamic forces induce structural motions of increasing amplitude. The airplane structure may consequently be designed for preventing occurence of flutter within the flight conditions likely to occur. If the required stiffness represents an unacceptable penalty, then active control of flutter, at least upon occurence of exceptional conditions, is necessary since flutter may cause catastrophic failure of a wing. Such active control has already been suggested. U.S. Pat. No. 3,734,432 discloses control of flutter with pairs of control surfaces for simultaneously countering the torsional and bending motions of the wing by applying torsional and bending forces to the wing with appropriate phases. Flutter is detected with sensors providing signals indicative of vibration amplitude, for instance accelerometers. Flutter cannot be damped with a single control surface and two mutually coupled control surfaces located at the leading edge and trailing edge, respectively, are required for creating torques and forces opposing the wing motion. Once particular mode only is corrected by each system and a number of sensors and flaps are required if a plurality of modes are to be damped.
Buffeting results in particular from unsteady separation arising on the wings and producing, through a coupling effect with the latter, vibrations of the structure of the aircraft. The phenomenon is not critical for the aircraft. However, it limits the flying domain of civil aircraft since the maximum level of buffeting is fixed by regulations. A reduction in the intensity of buffeting enables flight at a higher altitude and increase in the take off weight due to improved flight efficiency.
Buffeting is also detrimental to the flying performance of combat aircraft by limiting their manoeuvrability and their possibilities of firing.
Separations generating buffeting can appear at any flying speed. They are particularly accentuated at transonic speed since they are amplified by the oscillations of the shock waves. They also appear in flight at high angles of attack for a high value of the lift coefficient and on approaching stall conditions of the wings. Buffeting may also be caused by separations resulting from the airbrakes or spoilers used currently on aircraft to cause a loss in lift and rapid descent of the aircraft.
Buffeting is not due to regenerative oscillation of an unstable structure. It does not result into oscillations of ever increasing amplitude and does not represent an immediate structural hazard. It results from non-stationary flow separation occuring over limited areas on some parts of the wing. Separation takes place randomly and causes a forced oscillation of the wing at the natural vibration frequencies thereof. Oscillatory motion of the wing induces non-stationary pressure distributions on the whole of the wing and finally results in relatively low frequency, typically bending, oscillatory motion. A structure which is unstable will not exhibit buffeting, but rather flutter.
Buffeting is a phenomenon which has a large frequency spectrum. It may consequently be expected that control of buffeting will require detection in a large spectrum and filtering to retain only the oscillation modes to be dampened (rather than detection of an increasing oscillatory motion).
Attempts to reduce buffeting have up to now been made along two directions.
A first approach consists of increasing lift by a slight static action on the control surfaces.
A second approach consists of modifying the aerodynamic flow for avoiding separation by adapting the wing profile or providing vortex generators.
Such aproaches are difficult to implement. They only delay occurence of buffeting while they do not reduce its level. As a rule, they are only effective in respect of buffeting of a specific nature.
The inventors completely departed from the above approaches and explored a way which was apparently not possible in view of the broad frequency spectrum of buffeting; namely combatting the oscillatory effects of buffeting and damping the response of the structure. For that purpose, they had to analyze the separation phenomenon on a wing and to appreciate that there are in fact two effects:
a stationary phenomenon, which is indicated by a sharp change in the curve representing variation of the lift and of the moment vs. the angle of attack,
a non-stationary phenomenon, constituted by a zone of non-stationary fluctuating pressures covering a very wide frequency spectrum. The area of this zone extends over the upper wing surface and the intensity of the pulsations in the separations increases with the attack angle.
For slight separations, only the stationary phenomenon is appreciable since the non-stationary force created by the randomly varying pressures remains too weak to energize the structure.
However, if the separation zone extends, the twisting torque of the non-stationary forces generates a forced excitation of the wings which respond all the more as their own modes of vibration necessitate minimal energy.
These vibration modes create their own field of non-stationary pressures, extended this time to the whole of the wings, which facilitates in particular low frequency modes such as alternate flexion (bending) of the wing at the fundamental frequency of the latter.
It is an object of the invention to reduce buffeting and, thereby, to improve the performance and, possibly, the comfort of aircraft by a process usable whatever the origin of buffeting.
The invention is based on a different approach, fully adapted to aircraft provided with a generalized automatic control system consisting of limiting and/or countering the influence of non-stationary pressures due to the vibrational movement of the wings.